Image fusing assembly



June 23, 1970 R, T, HUGGlNs -ET AL 3,517,164

IMAGE FUSING ASSEMBLY Filed July 22, 1958 United States Patent O3,517,164 IMAGE FUSIN G ASSEMBLY Ralph T. Huggins, Glen Ellyn, andFrederick D. Meller,

Lombard, Ill., assignors to Addressograph-Multigraph Corporation, MountProspect, Ill., a corporation of Delaware Filed July 22, 1968, Ser. No.746,456 Int. Cl. H05b 1/00; F26b 3/04; G03d 3/12 U.S. Cl. 219-388 6Claims ABSTRACT F THE DISCLOSURE rThe application discloses a unit forfusing powder images on copy material using conduction of heat throughthe material. The unit includes a vacuum chest having a thermallyconductive wall over which the back of the copy material slides and towhich heat is supplied by electrical heaters carried thereon. The wallsare provided with spaced openings so that the moving copy material isheld in intimate engagement with the wall, and any air lm between thewall and the copy material is removed to permit an efficient heattransfer to the powder image by conduction from the wall and through thematerial.

BACKGROUND OF THE INVENTION This invention relates to copy makingapparatus of the type using fusible powder images and, moreparticularly, to a new and improved method of and apparatus for lixingor fusing powder images.

Photoelectrostatic machines for making one or more copies of graphicmaterial on sheet or web material form an electrostatic imagecorresponding to the graphic material which is then placed in visibleform by the application of a developer such as a toner material orelectroscopic powder. This toner material can be applied in variousmanners, as by a magnetic brush, cascade techniques, powder cloudgenerators, or from liquid systems, and is selectively attracted orrepelled by the electronic image to provide the visible image.Frequently, the toner material comprises electroscopic particles orpowder formed of a thermoplastic resinous material. After the powder ortoner image has been formed, it is lixed or placed in permanent form, asby fusion or solvent removal, incident to or after transfer to sheet orweb copy material.

-One common approach to lixing the powder or particle image relies onthe application of heat, and this heat is frequently applied to theimage bearing surface using radiation or convection techniques. Sincethe quantum of heat required for proper fusion tends to be constant fora given thermoplastic toner material, it becomes difficult to attainadequate fixing as the speed at which copies are made is increasedwithout operating the heating units at an undesirably high level orincreasing the the length of the travel path during which the image isexposed to heat. This increases the size of the machine and introducesthe possibility of damaging the copy.

SUMMARY Accordingly, one object of the present invention is to provide aphotoelectrostatic copy machine including new and improved llixing orfusing means.

Another object is to provide an apparatus for fusing powder images thatis compact and operable at acceptable temperature levels and is alsocapable of high speed image fusing.

A further object is to provide a powder image fusing apparatus in whichheat is transferred to the image largely by conduction.

3,517,164 Patented June 23, 1970 lv ce A further object is to provide apowder image fusing apparatus including a vacuum chamber having athermally conductive wall against which the back of the image bearingcopy material is held and to which heat for fusing the image is applied.

Another object is to provide a new and improved method of fusing powderimages on copy material in which the back of the copy material is heldin sliding contact with a heated and thermally conductive member by apressure differential.

In accordance with these and many other objects, an embodiment of theinvention comprises a vacuum chest or chamber having a thermallyconductive wall to which are afxed one or more electrical heatingelements. The wall is also provided with spaced perforations adjacentthe heating elements. The back of a copy sheet containing a powder imageis moved over the thermally conductive wall in sliding engagementtherewith, and the back of the sheet is held in intimate contact withthe thermally conductive wall by the pressure differential communicatedto the back of the sheet through the apertures. In this way, heat fromthe electrical heating elements is transmitted by conduction through thethermally conductive wall and the back of the sheet to fuse the powderimage. Using this method and apparatus, copy material speeds in therange of 15 to 50 feet per minute have been attained using thermoplasticpowders having softening points ranging from 70-190 C. It will beappreciated that even greater copy material speeds can be realized byoperating the system at the maximum temperature difference between thewall and softening point of the powder. For example, copy materialspeeds of 65 feet per minute and greater can be realized when using apowder that softens at about 70 C. and maintaining the wall at atemperature of 150 C.

BRIEF DESCRIPTION OF THE DRAWINGS Many other objects and advantages ofthe present invention will become apparent from considering thefollowing detailed description in conjunction with the drawings inwhich:

FIG. 1 is a top elevational View in partial section of a fusing assemblyembodying the present invention;

` and DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now morespecifically to the drawings, therein is illustrated a Ifixing or fusingunit or assembly which is indicated generally as 10 and which is capableof effecting the relatively high speed fusing of powder images carriedon web or sheet copy material 12. The powder image can be formeddirectly on the material 12 or transferred thereto, and in a preferredembodiment is produced on a photoconductive member and transferred tothe upwardly facing surface of the material 12 (FIG. 2) by an apparatusof the general type shown and described in detail in the copendingapplication of Loren E. Shelffo, Ser. No. 632,819, filed Apr. 21, 1967,which application is assigned to the same assignee as the presentapplica- Vacuum chest or chamber indicated generally as 18. The vacuumchamber 18 is illustrated as Irbeing formed of a single sheet of foldedmetal sealed along an edge 18A Y and closed at its opposite ends by theside walls 14 and 16 on which it is supported. The wall 14 is providedwith an opening 20 .by which the interior of the vacuum chamber 18 iscoupled to an air moving means or vacuum pump 22 by a section of duct24. Although the illustrated vacuum chamber 18 is illustrated as fbeingformed of sheet metal, other suitable materials could be used so long asa wall 18B or a suitable portion thereof is formed of a material havinggood thermal conductivity. The wall 18B is formed with a plurality ofrows of openings or apertures 26, 28, and 30 that are spaced from eachother in the direction of movement of the sheet or web material 12relative to the unit 10.

To provide a source of heat for fusing the powder image, the vacuumchest or chamber 18 is provided with electrical heating means includingtwo electrical heating units or assemblies 32 and 34. These two unitsare generally elongated and are secured to the lower surface of the wall18B disposed between and extending generally parallel to the rows 26,28, and 30- of openings or perforations. The heating units 32 and 34 canbe of conventional construction including a sole plate brazed or weldedto the wall 18B and carrying resistance elements on the oppositesurfaces thereof, or can comprise units in which the resistance elementsare cast into the sole plates included in the heating units 32 and 34.These units are energized from a suitable source of alternating currentenergy during periods of use of the copying machine of which theassembly forms a part and preferably are provided with adjustabletemperature regulating means of known construction. In one unit 10constructed in accordance with the present invention, the temperaturecontrol means can 'be regulated to energize the units 32 and 34 tomaintain the temperature of the thermally conductive wall 18B at atemperature in the range of 120- 150 C.

To provide means for feeding the sheet or web copy material 12 relativeto the wall 18B of the vacuum chamber or chest 18, two sets of rollers36 and 38 mounted on shafts 40 and 42, respectively, are provided. Thesets of rollers 36 and 38 are rotated in synchronism so as to advancethe material 12 over the heated wall 18B of the vacuum chamber 18 at thedesired speed and with the powder image facing outwardly or upwardlyaway from this wall. In one fusing unit 10 constructed in accordancewith the present invention, a variable speed drive (not shown) coupledto the shafts 40 and 42 can be adjusted to advance the material 12 at aspeed in a range of 15-50 feet per minute. As illustrated in FIG. 2 ofthe drawing, the sets of rollers 36 and 38 are offset vertically fromeach other so that the sheet or web material 12 is held in engagementwith the curved and somewhat up wardly inclined wall 18B of the vacuumchest 18.

When the fusing or fixing unit 10 is placed in operation, a controlcircuit (not shown) associated with the unit 10 or with the copyingmachine of which the unit 10 is a part is energized to initiate theoperation of the vacuum pump or air moving means 22 so that air iswithdrawn from the vacu-um chamber 18 through the opening 20 and theduct 24. Air is removed by the moving means 22 at a greater rate thanair can enter through the rows of openings 26, 28, and 30 so that apressure differential is maintained across the wall 18B. Theenergization of the control circuit also initiates the energization ofthe electrical heating units 32 and 34 so that the thermally conductivewall 18B or the thermally conductive portion thereof is heated to thedesired temperature under the control of the temperature regulatingmeans. This temperature can be adjusted in accordance with the type oftoner material to be fused by the unit 10. If desired, the controlcircuit for the electrical heating units 32 and 34 can lbe provided witha control which maintains the wall 18B at a temperature somewhat lowerthan that re quired for proper fusing during standby periods and iselevated to the proper fusing temperature when the unit 10 is actuallyplaced in use to x copies.

Powdered images are then produced on the copy material 12 eitherdirectly or, in the preferred embodiment, by transfer to the material 12in a machine of the type described in the above-identified copendingapplication of Shelffo et al. In this embodiment, the copy material 12preferably comprises plain or untreated paper, and the material 12 isfed to the nip between the inlet rollers 36 with the powder image facingupwardly (FIG. 2) or away from the wall 18B. The rollers 36 which wereplaced in operation together with the rollers 38 when the unit 10 wasplaced in operation feed the leading edge of the copy material 12against the upwardly inclined wall 18B and advance the leading edge overthe wall 18B to be received within the nip between the rollers 38, whichin turn feed the copy material to a discharge point.

As the copy material 12 moves over the wall 18B, it closes offcommunication between the atmosphere and the interior of the vacuumchamber 18` through the rows 26, 28, and 30 of perforations. Thisestablishes a predetermined pressure differential across the copymaterial 12 and holds it in intimate sliding engagement with the wall18B. This permits the heat stored in the wall 18B and in the heat sinksor sole plates associated with the electrical heating units 32 and 34 tobe transmitted by conduction through these thermally conductive partsand from the .back of the copy material 12 to the top or front surfaceon which the powder image is disposed. The use of the vacuum chest 18producing the pressure differential across the copy material 12 removesany film of air that exists between the back surface of the copymaterial 12 and the wall 18B and thus provides a good overall coeicientof thermal conductivity to the thermoplastic resin forming the tonerparticles. These particles are thus fused to fix or place in permanentform the image carried on the front or outwardly facing surface of thecopy material 12.

In one fixing or fusing unit 10 constructed in accordance with thepresent invention using a 'vacuum in the chest 18 on the order of 3-6inches of water, a toner material that fuses in the temperature range'between 70-115 C., a copy material feed rate in the range of 15-50 feetper minute, a plate 18B temperature in the range of 1Z0-150io C., it wasobserved that the toner material was fully fused to the copy material 12and that there was no evidence of discoloration of the copy material 12.The length of the wall 18B in this unit is approximately 3-5 inches, andthus the fusing unit 10I affords a very small or compact fusing unitwith a relatively short path of travel during which heat is applied butwhich is capable of attaining substantial paper feed rates.

Although the phenomena is not completely understood, it is believed thatthe improved transfer of heat to the powder image achieved with the unit10 results from the removal of any air film or boundary layer of airfrom the interface between the back of the copy material 12 and theadjacent surface of the wall 18B. It appears to be true that the thermalresistance R (resistance to transfer of heat from the wall 18B to thecopy material 12) approaches the relationship where dx is the thicknessof the boundary layer or film of air between the copy material 12 andthe thermally conductive wall .18B affording the heat source, K is aconstant for thermoconducti-vity of air which at one atmosphere variesfrom .014 to .017 with air temperatures between 32 F. and 200 F., and Ais the area of the copy material 12. From this relationship, it can beseen that if the thickness of the air film can be reduced to thegreatest extent possible, a corresponding reduction in the thermalresistance is effected, and the efiiciency of the heat transfer from thethermally conductive wall 18B to the copy material 12 is increased.

It will be appreciated that the significant advance made in thetechnique of fusing powder images by conductive heat transfer is thereduction in the air layer between the heating element and the imagecarrying support. The description of the invention sets forthtemperature ranges for the wall 18B and the softening point of thepowder material comprising the images and the speeds with which thefusing can occur. The speeds of fusing may vary widely in the instantinvention depending on the softening point of the thermoplastic powders,the thickness of the copy paper or material, and the temperature of thewall 18B under the inuence of the air suction system that tends todecrease the air layer.

Without the benefit of exhausting the air from between the copy materialand the wall 181B, the speed of fusion is greatly decreased, and thetemperature at which the wall 18B must be maintained is just below thescorching point of the copy material. Thus, the fusing system withoutthe vacuum means is insensitive to changes in the melting point of thetoner and the power input to the wall 18B in the sense that it is notpossible to use a lower melting point toner and obtain an accompanyingreduction in the heating power input or the temperature at which thewall 18B is held. This system lacking the vacuum means thus operates ina narrow range of speed and heat le'vel below which there is no fusion(only at very slow speeds) and above which further increases intemperature cause scorching or burning of the copy material. The systemincorporating the present invention is sensitive in the sense thatchanges in copy feeding speeds and temperature level are related and theuse of lower melting point toners permits a reduction in operatingtemperature or an increase in copy speed. Accordingly, the presentinvention results in a highly eicient system permitting both the use oflow wattage inputs with high speed fusion at temperatures well below thescorch point of paper and the use of thermoplastic materials having awide range of softening temperatures.

Although the present invention has been described with reference to asingle illustrative embodiment thereof, it should be understood thatnumerous other modifications and embodiments can be devised by thoseskilled in the art that will fall within the spirit and scope of theprinciples of this invention.

What is claimed and desired to be secured by Letters Patent of theUnited States is:

1. Apparatus for fixing thermoplastic powder images carried on onesurface of web or sheet material comprising:

stationary heating means for slidably :receiving said sheet materialthereon and applying heat by conduction to the surface opposite saidimage carrying surface,

transport means for slidably moving said sheet material across saidstationary heating means, and

means for removing the air between said heating means and the materialso that said sheet or web material is held in intimate contact with saidheating means as the material is transported thereacross.

2. An apparatus for fixing powder images carried on one surface of webor sheet material comprising:

an enclosure having a stationary wall portion of thermally conductivematerial and spaced apertures through said wall portion,

heating means for applying heat to said wall portion,

an air moving means coupled to said enclosure for moving air into saidenclosure through said apertures and for withdrawing air from saidenclosure, thereby to maintain a pressure differential along said wallportion,

and feeding means for feeding web or sheet material over said wall withsaid one surface facing away from said wall, said sheet oir web materialbeing held in intimate sliding contact with said wall by said air movingmeans so that heat from said heating means is transferred to said powderimage by conduction through the wall and the web or sheet material asthe latter is moved across said wall.

3. An apparatus for fusing powder images carried on one surface of webor sheet material comprising:

an enclosure having a stationary wall of thermally conductive material,said lwall having spaced openings therein,

electrical heating means carried on an interior surface of said wall forsupplying heat to said wall,

feeding means for feeding web or sheet material over an outer surface ofsaid stationary wall with said one surface of the sheet or web materialfacing away from said outer surface of said wall, and

air moving means coupled to the enclosure for producing a pressuredifferential across the wall and material to hold the material inintimate sliding contact with the wall as the material is transportedthereacross.

4. An apparatus for fusing toner images on one surface of web or sheetmaterial comprising:

a vacuum chamber having a stationary material engaging structure formedof thermally conductive material, said structure having spaced openingstherein,

electrical heating means coupled tothe structure for supplying heat tothe structure7 and material feeding means for feeding said materialrelative to the structure, said material being held against saidstructure during movement by the vacuum in the chamber so that heat istransmitted by conduction through the structure and the material to thetoner image. f

5. The apparatus set forth in claim 4 in which the structure includes lawall having spaced lines of openings,

and the electrical heating means are carried on the wall in the areasbetween the spaced lines of openings.

6. The apparatus set forth in claim 5 in which the 5 electrical heatingmeans includes spaced and elongated heating units extending generallytransverse to the direction of movement of the material relative to thestructure.

References Cited UNITED STATES PATENTS 1,575,366 3/1926 Johnson 34-2332,775,677 12/1956 sennetze 219-445 2,807,703 9/1957 Rennen 219-343 X2,807,704 9/1957 Auen et al. 219-343 2,821,612 1/1958 schuetze 219-4453,123,700 3/1964r Snyder e1 a1. 219-388 3,199,223 s/1965 Carlsen 34-1553,222,800 12/1965 Siegel et a1 34-212 3,397,303 8/1968 smith 219-388XVOLODYMYR Y. MAYEWSKY, Primary Examiner U.S. Cl. X.R.

